The present invention relates to a sheet feeder for feeding sheets of paper by means of feed roller pairs along a feed passage.
An image forming apparatus or another apparatus that provides processing for sheets of paper includes a feed unit, a sheet feeder, and a processing unit for forming an image on a sheet of paper. The sheet feeder has a feed passage leading from the feed unit to the processing unit. The sheet feeder includes a registration roller pair supported at the end of the feed passage.
Before the leading end of a sheet of paper reaches the registration roller pair, the rotation of the roller pair pauses. While the rotation is pausing, an angular adjustment is carried out to correct the angular position of the sheet with respect to the feed direction in which the sheet is being fed. Subsequently, the registration roller pair rotates to feed the sheet to the processing unit at such a timing that the leading end of the sheet registers with the leading end of an image formed in the processing unit. It is essential that the sheet feeder should feed sheets of paper with a space left between each sheet and the following sheet so as to allow time for carrying out an angular adjustment for each sheet at the registration roller pair.
In recent years, the processing speed of image forming apparatus has been increasing. In order for an image forming apparatus to print 40-60 sheets of paper per minute, its sheet feeder can feed sheets of paper with a space of 50-100 mm between each sheet and the following sheet. However, in order for the apparatus to print 100 or more sheets of paper per minute, the sheet feeder can feed sheets of paper with a space of only less than 50 mm between each sheet and the following sheet. In this case, it is difficult to allow time for carrying out angular adjustments at the registration roller pair of the apparatus.
The sheet feeder also includes a pickup roller and a parting roller. If the pickup roller picks up two or more sheets of paper at a time from the feed unit, the parting roller feeds one of them at a time, with the other sheet or sheets remaining at the parting roller. When the parting roller feeds the remaining sheet or one of the remaining sheets, the sheet starts to be fed from a position downstream from the feed unit of the apparatus. This results in advanced feed timing. If the pickup roller rotates at high speed, it may slip on a sheet of paper when picking up the sheet. This results in delayed feed timing.
The sheet feeder feeds sheets of paper at a preset speed along the feed passage. Advanced feed timing shortens the intervals between the sheets. The short intervals may make it impossible to allow time for carrying out angular adjustments at the registration roller pair. The short intervals may position the leading end of each sheet over or under the trailing end of the preceding sheet. Delayed feed timing widens the intervals between the sheets, thereby reducing the printing rate of the apparatus.
JP-H4-168076A discloses a conventional sheet feeder. In the acceleration range where this sheet feeder raises the feed speed at which it feeds sheets of paper, it raises the speed to a target speed while changing the acceleration state.
However, because the conventional sheet feeder changes the acceleration state under preset conditions, it is impossible for this feeder to change the acceleration state according to variable intervals between sheets of paper. This makes it impossible for the conventional sheet feeder to restrain the instability in the positions and directions in which it feeds sheets of paper, such as oblique feeding to a processing unit, paper jams, and feed speed drops.